U.S. patent number 6,673,287 [Application Number 09/858,919] was granted by the patent office on 2004-01-06 for vapor phase surface modification of composite substrates to form a molecularly thin release layer.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Tricia L. Breen, Laura L. Kosbar, Michael P. Mastro, Ronald W. Nunes.
United States Patent |
6,673,287 |
Breen , et al. |
January 6, 2004 |
Vapor phase surface modification of composite substrates to form a
molecularly thin release layer
Abstract
A method of exposing a composite organic/inorganic master to
alkylchlorosilanes in the vapor phase. Chlorosilanes participate in
facile reactions with hydroxyl groups existing on the surface of
inorganic oxides (such as glass or the native oxides on silicon,
aluminum, tin, etc.); or those in organics-containing phenolic or
alcoholic groups, such as photoresists. The alkyl group on the
silane can be chosen from a large selection of aliphatic or
aromatic organic groups that have substituents with varying
polarity and reactivity.
Inventors: |
Breen; Tricia L. (Hopewell
Junction, NY), Kosbar; Laura L. (Mohegan Lake, NY),
Mastro; Michael P. (Yorktown Heights, NY), Nunes; Ronald
W. (Hopewell Junction, NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
25329498 |
Appl.
No.: |
09/858,919 |
Filed: |
May 16, 2001 |
Current U.S.
Class: |
264/83; 264/102;
264/82; 427/133; 264/338; 264/219 |
Current CPC
Class: |
B82Y
40/00 (20130101); G03F 7/405 (20130101); B82Y
30/00 (20130101); G03F 7/0015 (20130101); B82Y
10/00 (20130101); G03F 7/0002 (20130101) |
Current International
Class: |
G03F
7/40 (20060101); G03F 7/00 (20060101); B28B
007/36 () |
Field of
Search: |
;264/82,83,225,226,102,338,219,39 ;216/41 ;430/324 ;427/133 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kuhns; Allan R.
Attorney, Agent or Firm: Beck; Thomas A. Morris; Daniel
P.
Claims
What we claim and desire to protect by Letters Patent is:
1. A method for creating a low surface energy, hydrophobic,
chemically bonded and molecularly thin release layer on masters
using vapor phase silanes that aid in the fabrication of a shaped
article of commerce from the master comprising: a substrate
composed of an organic composition containing hydroxyl
functionality, or a composite of organic and inorganic materials
having hydroxyl groups on the surface thereof; an organic
photoresist layer spun thereon, said photoresist layer having a
first and second surface; exposing and developing said photoresist
layer to produce images that are the inverse of those desired in
the final shaped article of commerce, said substrate and imaged
photoresist forming a master; placing said master into a chamber
and evacuating said chamber and master; heating said chamber to a
temperature in the range between room temperature and approximately
10.degree. C. and 20.degree. C. below the Tg of said organic
composition or photoresist; introducing silanes selected from the
group consisting of aliphatic, aromatic chlorosilanes, and alkyl
silane in the vapor phase into said chamber, so as to contact said
photoresist and said substrate surfaces which are exposed by
patterning said photoresist of said master, to form as a reaction
product a self-assembled mono-layer release layer on said exposed
substrate and photoresist surfaces; evacuating said silane from
said chamber; introducing a composition selected from the group
consisting of a curable thermosetting or a thermoplastic polymer
onto said master to conform to the shape of said resist features on
said master and placing said polymer in intimate contact with said
reaction product monolayer release layer coating said surfaces of
said master so as to form said shaped article of commerce; removing
said shaped article of commerce from said master.
2. The method of claim 1, wherein said master comprises a composite
structure containing a substrate of organic material, inorganic
material, or an organic/inorganic composite material, and a
patterned organic layer.
3. The method of claim 2, wherein said inorganic material is an
inorganic oxide selected from the group consisting of: silicon
dioxide, aluminum oxide, tin oxide, glass, and quartz.
4. The method of claim 2 wherein said substrate comprises an
organic or material or organic/inorganic composite selected from
the group consisting of: polymers containing hydroxyl functional
groups such as epoxy resins, phenol-formaldehyde resins,
polysaccharides, and copolymers containing monomers such as vinyl
alcohol and methacrylic acid, or polymer composites where the
polymeric component contains hydroxyl functional groups such as
epoxy fiberglass composites or reinforced phenolic resins.
5. The method of claim 2, wherein said patterned organic layer is
selected from the group consisting of: a short-chain organic
oligomer having less than ten monomer units, a long-chain organic
polymer having at least ten monomer units, a photoresist and
mixtures thereof.
6. The method of claim 5, wherein said photoresist is a
phenolic-based positive or negative working photoresist.
7. The method of claim 6, in which the organic material substrate
is patterned.
8. The method of claim 7, wherein the patterned substrate is used
as the master for formation of an elastomeric stamp.
9. The method of claim 1, wherein said silanes are in the vapor
phase and are brought into contact with said substrate.
10. The method of claim 9, wherein said aliphatic silanes contain
at least 12 carbon atoms.
11. The method of claim 9 wherein the aliphatic or aromatic group
attached to the silane is partially or completely fluorinated and
contains containing at least 6 carbon atoms with at least two
carbon atoms fully fluorinated, or a fully fluorinated phenyl
silane.
12. The method of claim 11, wherein said silane is
(tridecafluoro-1,1,2,2-tetrahydrooctyl) trichlorosilane.
13. The method of claim 10, wherein said evacuated chamber is
heated to increase the reaction rate of the silane with said
substrate.
14. The method of claim 1 wherein said vapor silane-treated master
is brought into contact with a thermosetting or thermoplastic
polymer to replicate the master.
15. The method of claim 14, wherein said polymer is
polydimethylsiloxane.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the fabrication of
stamps for microcontact printing using a master that contains
surfaces of organic materials or composites of organic and
inorganic materials, and more particularly a method of modifying
the chemical nature of the surfaces of the master through a vapor
phase treatment to improve the ease of separation of the stamp from
the master.
2. Description of Related Art
Micromolding and microcontact printing as pattern transfer
techniques are areas of significant interest for a wide range of
applications, especially in the microelectronics industry.
Microcontact printing, known as "stamping", techniques are under
development as low-cost alternatives to lithography for pattern
definition on cost sensitive applications such as flat panel
displays. Stamps made from siloxane polymers (such as
poly(dimethylsiloxane) (PDMS)) are commonly used due to their ease
of fabrication, conformal nature that allows stamping over
topographical features, and potential for repeated use to pattern
multiple substrates. Stamping has been demonstrated to transfer
very thin coatings, down to monolayer coverage, of organics onto
surfaces to act as etch barriers, plating seed layers, or
ultra-thin patterns that can modify the interaction of the surface
with subsequent coatings of organics--either enhancing or resisting
deposition of materials in specific areas. Imaging with siloxane
stamps has been demonstrated down to submicron dimensions.
In molding and stamping applications, a curable (thermosetting)
material, such as PDMS, or a pliable (thermoplastic) material such
as a resist is formed into the desired shape by bringing it in
contact with a rigid pre-formed master. Thermoplastic materials are
generally cast from a solvent or heated until pliable and conformal
with the master. Thermosetting materials are generally composed of
monomers or polymer precursors that generally have low viscosity
prior to curing (crosslinking), and conform easily to the surface
of the master. Curing at room or elevated temperatures or under
exposure to an appropriate source of radiation causes crosslinking
of the polymer precursors, which will be transformed into an
elastic or inelastic solid. To achieve an accurate replication of
the master, the polymeric material must be in intimate contact with
the surfaces of the master.
After the thermosetting or thermoplastic stamp has been formed to
replicate the shape of the master, it must be separated from the
master without causing damage to either the stamp or the master.
The cured/formed polymer will have reduced flexibility, and may
adhere to, or even be chemically bonded to the surface of the
master. Adhesion of the stamp to the master increases the
difficulty in separating the stamp from the master, and may result
in fractures or tears in the polymeric stamp, with the torn
sections of the stamp adhering to the master. Tear-outs result in
stamps that are imperfect replicas of the master, and are unusable
in manufacturing applications that require defect free
reproductions. Tear-outs also reduce the utility of the master for
production of future articles.
The master itself may be fabricated from a range of materials,
although they are generally composed of inorganic materials, such
as glass or silicon, or composite structures of an organic material
on a glass or silicon substrate, although all organic masters are
also feasible. Inorganic masters are generally produced by
lithographically patterning a photoresist that has been spun onto
the substrate. The exposed and developed photoresist creates a
relief pattern that is the inverse of the desired stamp pattern.
The patterns on the master may be as small as micron or sub-micron
dimensions in width, but are usually several microns deep. Creating
relief structures in a silicon or glass substrate requires
transferring the resist pattern via wet chemical or plasma etching
of the substrate. Such substrates can easily be treated with
standard silane or fluorosilane solutions to increase the
hydrophobicity of the surface and improve the release of organic
materials molded from the master, as described in U.S. Pat. Nos.
5,425,848, 5,817,242 and 6,027,595. Release layers such as the
silanes used in these patents are applied in dilute solutions of
appropriate nonpolar organic solvents such has alkanes, chlorinated
or fluorinated solvents, etc. These solvents provide adequate
wetting and minimal interaction with inorganic masters, such as
those made of silicon or glass.
A less time-consuming and more cost-effective technique for
preparation of masters, also in common usage, is the fabrication of
composite masters with a permanent patterned photoresist layer on a
glass or silicon substrate. Using this fabrication technique, the
exposed and developed resist images form the topographical features
that will be replicated in the elastomeric stamp. No mention is
made in the literature of treating such masters with release
agents. The common solvents for silane release agents produce
swelling or distortion of organic films, such as photoresists.
Interactions with the organic features on the master limit the
utility of solvent applied silanes to produce release layers on
organic or composite organic/inorganic masters.
Other examples of the fabrication and uses of microcontact printing
stamps and masters include U.S. Pat. No. 5,512,131, which describes
the fabrication of elastomeric stamps and the use of these stamps
to transfer self-assembled monolayers ("SAM") of molecular species
onto a solid substrate; and U.S. Pat. No. 5,900,160 describes the
etching of said substrates after transfer of the SAM using an
elastomeric stamp. U.S. Pat. No. 5,817,242 describes the use of a
deformable layer as part of the stamp to accommodate for unevenness
of the substrate being stamped. Fabrication of stamps in this
patent includes transfer of resist features from one inorganic
substrate to another.
U.S. Pat. Nos. 5,425,848 and 6,027,595 demonstrate the use of
stamps to produce patterned resist images using molding techniques;
U.S. Pat. No. 5,925,259 employs stamps to provide patterned
"microcontainers" for etchants or reactants/catalysts to interact
with the substrate in selected areas.
SUMMARY OF THE INVENTION
The present invention comprises exposing a composite
organic/inorganic master to alkylchlorosilanes in the vapor phase.
Chlorosilanes participate in facile reactions with hydroxyl groups
existing on the surface of inorganic oxides (such as glass or the
native oxides on silicon, aluminum, tin, etc.); or those in
organics-containing phenolic or alcoholic groups, such as
photoresists. The alkyl group on the silane can be chosen from a
large selection of aliphatic or aromatic organic groups that have
substituents with varying polarity and reactivity. The preferred
materials to increase the hydrophobicity of the master and minimize
adhesion of the stamp are fluorinated aliphatic chlorosilanes with
at least eight carbon atoms, or long chain alkyl silanes with at
least twelve carbon atoms, although aromatic silanes, such as
phenyl silane are also applicable. To avoid swelling or distortion
of the organic features on the master that can be caused by
exposure to solvents, the chlorosilanes were brought into contact
with the master in an evacuated, heated chamber, resulting in
reaction of the silanes with all surfaces of the master to produce
uniform, hydrophobic surfaces. The temperature of the reaction
chamber affects the rate of reaction of the silane with the organic
and inorganic surfaces of the master. Since the silanes chemically
bond to the surfaces of the master, the hydrophobicity of the
surface is retained for preparation of multiple stamps, increasing
the useful lifetime (and so lowering the effective cost) of the
master.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(a-g) illustrates the process of master and stamp formation
using the disclosed method for vapor phase application of silanes
to the master as a release layer prior to stamp fabrication.
FIGS. 2(a-c) Scanning electron micrographs (SEMs) that illustrate
PDMS tear-out associated with untreated masters.
FIGS. 3(a-f) SEMs of resist images exposed to organic solvents
commonly used for silane application that illustrate the swelling,
distortion, and loss of adhesion of the resist features that can
accompany this method of silane application.
FIGS. 4(a-d) SEMs of resist features before and after vapor
fluorosilane treatment that illustrate that no change occurs in the
resist image due to the vapor silane treatment.
DETAILED DESCRIPTION OF THE INVENTION
The present invention describes a method for applying a
self-assembled monolayer of a release agent to composite
organic/inorganic substrates, specifically substrates used as
masters for the preparation of elastomeric stamps for microcontact
printing. The release layer aids in removal of the elastomeric
stamp from the master by decreasing the interaction of the
elastomer with the surfaces of the substrate. This reduces damage
to both the stamp and the master during the process of
separation.
Referring to FIG. 1, a schematic illustration of vapor treatment of
a master as part of the process of stamp fabrication is presented.
The substrate, 22 (FIG. 1a) is composed of glass, silicon, or a
metal with oxide-containing hydroxyl groups on its surface 24. This
substrate could also be composed of a rigid or flexible organic or
composite material that contains hydroxyl functionality. As
illustrated in FIG. 1b, a layer of photoresist 26 is spun on the
surface 24 of the substrate, and the photoresist is exposed and
developed to produce images that are the inverse of those desired
in the final stamp.
The composite of the patterned photoresist on the substrate, which
shall be referred to hereinafter as the "master", is placed in a
chamber that is then evacuated. The chamber may be heated above
room temperature, with the temperature range generally limited to a
maximum of 10-20.degree. below the glass transition temperature of
the photoresist to minimize deformation of the surfaces of resist
images 28. In certain cases, temperature induced deformation of
resist images 28 may be desirable, such as to tailor the sidewall
slopes of the resist. In these cases, increasing the chamber
temperature above the glass transition temperature of the resist
will allow a degree of controlled deformation.
Once the chamber with the master has been evacuated, silane vapor
30 is introduced into the chamber as illustrated in FIGS. 1c-d,
generally through opening a valve to a container of the liquid
silane. The partial pressure of silane 30 in the chamber may be
controlled by the inherent vapor pressure of the selected silane,
the temperature of the container in which the silane is held, the
vacuum in the chamber, and combinations thereof.
The specific silane may be chosen, according to the present
invention, to optimize the desired properties of the surface and
accommodate any limitations on processing conditions. The silanes
are composed of a head, or reactive group, and a tail group that
affects the hydrophobicity and surface free energy of the surfaces
to which the silane is bound. For the purposes of this invention,
desirable head groups include trichlorosilane,
methyldichlorosilane, dimethylchlorosilane, trimethoxysilane, or
triethoxysilane. The specified head groups will react with hydroxyl
groups on the surface of inorganic substrate 24 and hydroxyl or
phenolic groups on the surface of resist 28. For the purposes of
this invention, desirable tail groups include alkanes, especially
linear alkanes with more than 12 carbons, aromatic groups, and
aromatic or aliphatic groups with fluorine substitution, especially
linear alkanes with the terminal hydrocarbon(s) completely
fluorinated. Highly fluorinated alkanes such as
(tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane or
(heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane produce
particularly highly hydrophobic surfaces on both the organic and
inorganic surfaces of the master.
The vapor phase silanes 30 undergo a facile reaction with the
surfaces of photoresist 28 and substrate 24 producing monolayer
coverage of the silane on the organic 32 and inorganic 34 surfaces
of the master (FIG. 1e). The silanes form a covalent bond with the
hydroxyls on the organic and inorganic surfaces, creating a stable
hydrophobic surface. The isotropic nature of the contact of the
silane vapor with all surfaces on the master produces uniform
coverage independent on the orientation of, or features on the
substrate. The contact time required to achieve full coverage of
the surfaces varies with the choice of silane and the temperature
of the chamber; but it has been measured to be anywhere from one
minute to several hours. Once a uniform monolayer of silane has
reacted with the surface, introduction of silane into the chamber
ceases and the remaining silane is evacuated prior to removal of
the master.
Elastomeric stamps for microcontact printing fabricated by coating
the master as indicated in FIG. 1(f) with polymeric precursors,
such as siloxanes, and curing the precursors using heat or
irradiation to form crosslinked structure 36. The surface of
elastomeric stamp 38 is in contact with the silane monolayer, which
has minimal interactions with the cured polymer as compared with
untreated resist or inorganic substrates. The limited interactions
between the elastomer and the master result in minimal forces
required to separate the stamp from the master (FIG. 1g), thus
reducing defect formation due to adhesion of the elastomer to the
master.
The invention herein disclosed eliminates several problems with the
production of elastomeric stamps for microcontact printing that
would otherwise limit their utility in manufacturing applications.
As mentioned previously, tear-outs in the elastomer as it is
removed from contact with the master often leave pieces of the
elastomer adhering to the master (as shown in the SEMs in FIG. 2),
which create defects in both the stamp and the master that render
them useless. To date, most stamping demonstrations have been
accomplished on small, laboratory scale parts where the
manufacturing issues of defect free stamps, reproducibility, and
reusability of masters have not been primary concerns. As larger
stamps, up to 1 meter.sup.2 in size, are being developed for flat
panel applications the adhesion of the PDMS to the master, and the
defects created by this adhesion severely limit manufacturability.
The surface treatment disclosed in the present invention reduces
the interaction between the elastomeric stamp and the master to a
level that requires minimal force for separation, and eliminates
tear-outs in the stamp. The process described in the present
invention allows the production of stamps free of tear-out defects
and improves the potential for reuse of masters.
The traditional solution-based silane treatments used for inorganic
masters are incompatible with the less expensive organic/inorganic
masters required for cost-effective manufacturing processes. FIG. 3
illustrates the deformation and swelling of photoresist features
subjected to solution-based silane treatments. The vapor phase
contact with silanes described in this invention avoids any
deformation or distortion of the resist images, as demonstrated in
the SEM's of untreated and vapor treated resist images depicted in
FIG. 4
EXAMPLE 1
A master substrate having an exposed and developed photoresist
pattern was fabricated by photolithography. The master was placed
in a chamber that was heated to 40.degree. C. and evacuated to 2
torr.
A valve to a container of liquid
(tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (TFOTS) was
opened, allowing vapor of the TFOTS to enter the evacuated chamber
and react with the substrate. The chlorosilyl head group undergoes
a facile reaction with hydroxyl groups on the surface of the glass
as well as with the hydroxyl or phenyl groups in the resist. The
fluorinated tail portion of the silane extends outward from the
surface of the substrate, increasing the hydrophobicity of both
inorganic and organic surfaces. The substrate remained in the
evacuated chamber in contact with the TFOTS vapor for 60 minutes,
after which the valve to the TFOTS container was closed, and the
chamber was flooded with nitrogen. The chamber was evacuated again,
and refilled with nitrogen before removing the master.
The contact angle of static drops of water on the surface of both
the resist and the glass portions of the master were measured for
untreated and fluorosilane-treated masters. Large contact angles
with water indicate high hydrophobicity and low surface free
energy, which are associated with reduced chemical interactions and
adhesion. The contact angle of water on the resist increased from
75.degree. on the untreated sample to 95.degree. on the treated
sample. The contact angle of water on the glass portions of the
master increased from 40.degree. on the untreated sample to
90.degree. on the treated sample. The fluorosilane treatment
significantly increases the hydrophobicity of the glass portions of
the master, as well as moderately increasing the hydrophobicity of
the resist portions. Perhaps more importantly, the fluorosilane
treatment produces a uniform degree of hydrophobicity between the
two disparate materials.
A 10:1 (w:w) mixture of PDMS-Sylgard Silicone Elastomer 184 and
Sylgard Curing Agent 184, a poly(dimethyl siloxane) precursor and
crosslinking agent (Dow Corning Corp. Midland, Mich.) was degassed
under vacuum for about 10 minutes, then the mixture was poured over
the master. The PDMS cured at 65.degree. C. within 60 minutes to
produce an elastomeric stamp. After cooling to room temperature,
the PDMS stamp was peeled from the master. Very little, if any
force was required to remove the stamp from the template after the
master was treated with the fluorosilane vapor, and there was no
evidence of PDMS adhering to the master or tear-outs in the
stamp.
EXAMPLE 2
A master substrate having an exposed and developed photoresist
pattern was fabricated by photolithography. The master was placed
in a chamber that was heated to 80.degree. C. and evacuated to 2
torr. A valve to a container of liquid TFOTS was opened. The
substrate remained in the evacuated chamber in contact with the
TFOTS vapor for 10 minutes, after which the valve to the TFOTS
container was closed, and the chamber was flooded with nitrogen.
The chamber was evacuated again, and refilled with nitrogen before
removing the master. The contact angles of the resist and glass
after this treatment averaged 94.degree. and 92.degree.
respectively. The PDMS stamp was prepared as described in Example
1, and the stamp was easily removed from the master with no
evidence of PDMS adhering to the master.
EXAMPLE 3
A master substrate having an exposed and developed photoresist
pattern was fabricated by photolithography. The master was placed
in a chamber at room temperature that contained 0.1 ml of
octyltrichlorosilane in an open glass vial and the chamber was
evacuated to about 10 torr. The substrate remained in the evacuated
chamber in contact with the octyltrichlorosilane vapor overnight.
The contact angle of water on the glass and the resist after this
treatment averaged 98.degree.. The PDMS stamp was prepared as
described in Example 1, and the stamp was easily removed from the
master with no evidence of PDMS adhering to the master.
* * * * *